Wireless personal area networks (WPAN) are those networks generally used for interconnecting devices centered around people where the connections are wireless. Because most personal area networks are wireless, the acronym WPAN and the term “wireless network” often are considered to be virtually synonymous. Generally, a wireless personal area network uses technology that permits communication over a very short range, typically 10 meters or less. One common example of this technology is 802.15.4, which is a standard developed by the Institute of Electrical and Electronics Engineers (IEEE).
As is well known in the art, a WPAN can serve to interconnect all the ordinary personal computing and communicating devices that many people carry with them today. Moreover, WPAN can also serve a more specialized purpose such as allowing a surgeon and other medical team members to communicate during an operation. A key concept in WPAN technology is known as “plugging-in.” In the ideal scenario, when any two WPAN-equipped devices come into close proximity (within several meters of each other) or within a few kilometers of a central server, they can communicate as if connected through a wired connection. Still another important feature of WPAN is the ability of each device to selectively lock out other devices, preventing unwanted interference or unauthorized access to information.
Currently, technology for WPAN devices and systems is in its infancy and is undergoing rapid development with a proposed operating frequency at approximately 2.4 GHz in digital modes. The ultimate objective of this technology is to facilitate seamless operation among home or business devices and their networking systems. In an ideal scenario, every device in a WPAN shall be able to plug in to any other device in the same WPAN, provided they are within physical range of one another. In addition, WPANs worldwide shall be interconnected. As one example, an archeologist on site in Greece might use a personal digital assistant (PDA) to directly access databases at the University of Michigan in Ann Arbor, Mich., and to transmit findings to that database.
Radio frequency (RF) technology enabling WPAN-equipped devices to interconnect can be very complex. Operation at frequencies at and above 1 GHz requires specialized RF circuit topologies for fast and reliable operation. One such circuit topology that can present a problem at these frequencies is the voltage controlled oscillator or “VCO.” As seen in prior art FIG. 1, a block diagram of the VCO typically is arranged in a loop configuration where a series of delay cells are used to offer both gain and phase delay. As is well known by those skilled in the art, this type of configuration is also commonly referred to as a “ring oscillator.” The delay cells are used to provide both in-phase (I) and quadrature (Q) digital output signals at some time later than the input signal applied to the VCO. The output signal is inverted and fed back to the input of the delay cells. This in turn causes the circuit to oscillate in view of a 180 degree phase shift between input and output.
The topology of the VCO delay cell has offered interesting challenges when requiring it to operate at frequencies above 1 GHz. Prior art FIGS. 2 and 3 illustrate both a current starved ring oscillator delay cell and a non-linear resistive capacitive (RC) type delay cell. Both of these types of circuit topologies operate in a voltage mode controlled by the voltage gain (Gm) of the amplifier used in their respective circuits. These commonly used delay cell topologies are adequate for frequencies under 1 GHz, however, the VCOs used with these types of delay cells do not offer adequate frequency range when operating on or around 2.4 GHz. A VCO such as that shown in FIG. 3 will have a small to medium level of tuning range because of the limited resistive load variation set by the metal oxide semiconductor field effect transistor (MOSFET) operating in the triode region. By way of example, U.S. Pat. No. 6,011,443 shows a complementary metal oxide semiconductor (CMOS) VCO that includes a voltage-to-current converter for generating reference currents. This type of circuit causes each of the load metal oxide semiconductor (MOS) transistors to operate in the triode region and suffers from all of the inherent drawbacks mentioned herein. Moreover, tank oscillators which require an inductor and capacitor to oscillate (LC type) take up large amounts of circuit area when implemented with current integrated circuit (IC) technology, and generally require additional processing steps during the manufacturing process.
Thus, the need exists to provide a new circuit topology for a high frequency VCO using delay cells operable at frequencies in the WPAN IEEE 802.15.4 standard. The new invention should be capable of being implemented in an all CMOS technology operable on or about 2.4 GHz. The device should use no internal or external inductors that would require additional cost and IC surface area. Moreover, the VCO should be capable of tuning over all process, temperature, and supply voltage corners such that its operational voltage remains within a few decibels (dB) of a nominal value.